July 18, 2023
Have you ever experienced that after installing camera with great difficulty, there is no reference image when you turn on power? Check for problems with settings of software, camera itself and switch, then pay attention to power supply and wiring.
There is a problem with line, sometimes even if you have an engineering treasure, it may not be useful! You should use most reliable tool, multimeter.
Today we will talk about how a multimeter determines location of a cable break. While it is very unlikely that you will encounter breakpoints in new projects, you are very likely to encounter them when fixing old monitoring projects.
Digital multimeters are now commonly used. We previously reported that use of multimeters is not covered here.
The difficulty in checking line break point is that line break point is only inside line, and outer insulation is wrapped, making it difficult to determine exact position of broken line.
How to use a multimeter to determine cable break point?
Some say you can measure resistance directly. It may be, but once measured, it doesn't work! You have to constantly cut cable to eliminate it step by step. Not to mention trouble, when it was checked, cable was torn into 18 segments!
The correct method for checking cable break points is inductive and capacitive.
Induction method:
2. Select a well-insulated core in cable, connect 220 V AC phase line (live line) and do not ground line;
3. If you are using an induction electric pen, touch inductive contact on electric pen with your finger and check if electric pen works normally outside insulating layer of charged body. If you are using a digital multimeter, set multimeter in 20 or 200 mV range, put a thin plastic insulating sleeve over red probe, grasp black probe with your hand, check outside insulating layer of a transparent charged body, and read; then stand back The charged body is far away and read; compare difference between two readings, usually there should be a higher value on charged body, such as 0.4mV, and a lower value away from charged body, such as 0.15mV, remember this function, you can start testing.
4. Check cable next to cable. When induction pen indicator dims or multimeter reading drops significantly, change is a breakpoint.
5, test is over, pay attention to discharge.
Capacitive method:
1. Hang all insulated core conductors and armor layers at both ends of cable;
2. Measure capacitance value of intact insulating core and insulating core with broken core against steel strip (or third intact insulating core) at both ends and record value; at this time, two ends of corresponding intact insulating core. The measured capacitance value must be very close; sum of capacitance values at both ends of same damaged core should be slightly greater than capacitance value of undamaged insulating core at same location, indicating that there is only one break point or multiple break points but very close together ; if same breakpoint If sum of capacitance values at both ends of strand is less than capacitance value of undamaged insulating strand at same location, then there are at least two breakpoints;
3. According to comparison and calculation of capacitance value of damaged core insulating core and undamaged insulation core, lengths of both ends are obtained respectively. At this time, length may differ from actual length, and next step is to recalibrate.
4.If sum of effective lengths is greater than actual length, excess length value is negative, and if less than actual length, it is positive, then use capacitance value of broken core insulation core to divide difference and get a long section Correct long segment, change short segment and get actual breakpoint position.
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